<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes">
<style>

canvas { border: 1px solid black; }


</style>
</head>
<body>

<script src="https://twgljs.org/dist/4.x/twgl-full.min.js"></script>
<script id="vshader" type="whatever">
    attribute vec4 a_position;
    attribute float a_lengthSoFar;
    varying float v_lengthSoFar;
    void main() {
      gl_Position = a_position;
      v_lengthSoFar = a_lengthSoFar;
    }    
</script>
<script id="fshader" type="whatever">
precision mediump float;
varying float v_lengthSoFar;
uniform sampler2D u_pattern;
#define NumDashes 6.0
void main() {
    gl_FragColor = texture2D(
      u_pattern, 
      vec2(fract(v_lengthSoFar * NumDashes)), 0.5);
}
</script>
<canvas id="c" width="300" height="300"></canvas>


</body>
<script>

const gl = document.querySelector("#c").getContext("webgl");

// Note: createProgramFromScripts will call bindAttribLocation
// based on the index of the attibute names we pass to it.
var program = twgl.createProgramFromScripts(
    gl, 
    ["vshader", "fshader"], 
    ["a_position", "a_lengthSoFar"]);
gl.useProgram(program);

function distance(array, ndx1, ndx2) 
{
  ndx1 *= 3;
  ndx2 *= 3;

  var dx = array[ndx1 + 0] - array[ndx2 + 0];
  var dy = array[ndx1 + 1] - array[ndx2 + 1];
  var dz = array[ndx1 + 2] - array[ndx2 + 2];

  return Math.sqrt(dx * dx + dy * dy + dz * dz);
}

// Used this line in the console to generate the positions
// var sub = 10; for (var ii = 0; ii <= sub; ++ii) { r = (ii & 1) ? 0.5 : 0.9; a = ii * Math.PI * 2 / sub; console.log((Math.cos(a) * r).toFixed(3) + ", " + (Math.sin(a) * r).toFixed(3) + ", "); }

var positions = [
  0.900, 0.000, 0,
  0.405, 0.294, 0,
  0.278, 0.856, 0,
  -0.155, 0.476, 0,
  -0.728, 0.529, 0,
  -0.500, 0.000, 0,
  -0.728, -0.529, 0,
  -0.155, -0.476, 0,
  0.278, -0.856, 0,
  0.405, -0.294, 0,
  0.900, -0.000, 0,
];
    
var lengthSoFar = [0];  // the length so far starts at 0
for (var ii = 1; ii < positions.length / 3; ++ii) 
{
  lengthSoFar.push(lengthSoFar[ii - 1] + distance(positions, ii - 1, ii));
}

var vertBuffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, vertBuffer);
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(positions), gl.STATIC_DRAW);
gl.enableVertexAttribArray(0);
gl.vertexAttribPointer(0, 3, gl.FLOAT, false, 0, 0);

var vertBuffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, vertBuffer);
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(lengthSoFar), gl.STATIC_DRAW);
gl.enableVertexAttribArray(1);
gl.vertexAttribPointer(1, 1, gl.FLOAT, false, 0, 0);

// Since uniforms default to '0' and since 
// the default active texture is '0' we don't
// have to setup the texture uniform. 
var pixels = [
    0, 0, 255, 255,
    0, 0, 255, 255,
    0, 0, 255, 255,
    0, 0, 255, 255,
    0, 0, 0, 0,
    0, 0, 0, 0,
    255, 0, 0, 255,
    0, 0, 0, 0,
    0, 0, 0, 0,
    255, 0, 0, 255,
    0, 0, 0, 0,
    0, 0, 0, 0,
];

var tex = gl.createTexture();
gl.bindTexture(gl.TEXTURE_2D, tex);
gl.texImage2D(
    gl.TEXTURE_2D, 0, gl.RGBA, pixels.length / 4, 1, 0,
    gl.RGBA, gl.UNSIGNED_BYTE, new Uint8Array(pixels));
gl.texParameteri(
    gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
gl.texParameteri(
    gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.LINEAR);
  gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.CLAMP_TO_EDGE);
  gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.CLAMP_TO_EDGE);

gl.drawArrays(gl.LINE_STRIP, 0, positions.length / 3);


</script>
